Optimize your gravimetric feeding process and refill strategies
The desired mass flow rate of material leaving the feeder.
Please enter a valid positive feed rate.
Density of the material in loose bulk form (e.g., pellets, powder).
Please enter a valid positive density.
Total internal volume of the weigh hopper.
Please enter a valid positive volume.
Rate at which the refill device fills the hopper (must be > Feed Rate).
Refill rate must be greater than feed rate.
Percentage of capacity where refill triggers.
Percentage of capacity where refill stops.
High level must be greater than low level.
Time Between Refills (Gravimetric Mode)
— min
Duration the feeder operates in true gravimetric mode before needing a refill.
— sec
— kg
— cycles/hr
— kg
Cycle Analysis Table
Parameter
Value
Unit
Hopper Weight vs. Time (Simulation)
— Weight Profile |
— High Limit |
— Low Limit
What are Loss-in-Weight Feeder Calculations?
Loss-in-weight feeder calculations are the mathematical processes used to size, configure, and optimize gravimetric feeding systems in industrial manufacturing. A loss-in-weight (LIW) feeder operates by weighing the entire feeding system (hopper, material, and metering device) and measuring the rate at which weight decreases over time.
These calculations are critical for process engineers to ensure that the feeder can maintain the target mass flow rate without interruption. By accurately calculating variables such as refill frequency, usable hopper capacity, and refill duration, manufacturers can prevent process instability, material bridging, or "starving" the downstream extruder or mixer.
Common industries utilizing these calculations include plastics compounding, food processing, pharmaceutical manufacturing, and chemical processing, where precise ingredient ratios are mandatory for product quality.
Loss-in-Weight Feeder Formula and Explanation
The core principle of a loss-in-weight feeder is derived from the change in weight ($\Delta W$) over a change in time ($\Delta t$). The controller adjusts the speed of the metering device (screw or vibratory tray) to keep this rate constant.
Core Formula
The mass flow rate ($\dot{m}$) is calculated as:
Rate (kg/hr) = (Weight at Time T1 – Weight at Time T2) / (T2 – T1)
Refill Logic Formulas
Since the hopper has a finite capacity, it must be refilled periodically. The system typically switches to "volumetric mode" during refill. The key calculations for sizing the cycle are:
A plastics manufacturer needs to feed polyethylene pellets into an extruder at 100 kg/hr. The pellets have a bulk density of 550 kg/m³. They use a 200-liter hopper with refill setpoints at 20% and 80%.
Total Capacity (Mass): 200 L × 0.55 kg/L = 110 kg
Usable Batch: 110 kg × (0.80 – 0.20) = 66 kg
Time Between Refills: 66 kg / 100 kg/hr = 0.66 hours (39.6 minutes)
Interpretation: The system runs stably for nearly 40 minutes between refills, which is excellent for accuracy.
Example 2: Fine Powder Additive
A chemical process requires a fine powder additive at 20 kg/hr. The density is light (300 kg/m³). The hopper is small (50 Liters). The refill source is slow, only providing 40 kg/hr.
Interpretation: This setup is problematic. The refill takes 27 minutes, during which the feeder is in volumetric mode (blind). The refill source rate should be increased significantly to minimize blind time.
How to Use This Loss-in-Weight Feeder Calculator
Enter Target Feed Rate: Input the required process rate in kg/hr.
Input Material Properties: Enter the bulk density. If unknown, weigh a known volume (e.g., a 1-liter cup) to estimate.
Define Hopper Geometry: Input the total volume of the hopper in Liters.
Set Refill Parameters: Input the rate at which your refill system (vacuum loader, slide gate) delivers material.
Adjust Setpoints: Define the Low and High level percentages. Standard practice is often 20% Low and 80% High.
Analyze Results: Look at the "Refill Duration." If this time is too long relative to the "Time Between Refills," your accuracy may suffer.
Key Factors That Affect Feeder Results
Several physical and operational factors influence the accuracy of loss-in-weight feeder calculations:
Material Bulk Density Variations: If density changes during the process, the volumetric backup mode during refill will be inaccurate.
Refill Rate Speed: A slow refill extends the time the feeder operates without gravimetric feedback. Ideally, refill should be 10x faster than the feed rate.
Venting and Air Pressure: Rapid refills can aerate powders, causing them to flush through the screw like a liquid. Proper venting is essential.
Vibration Interference: External vibrations can corrupt the weight signal, causing the controller to react to noise rather than weight loss.
Material Bridging: If material sticks to the hopper walls (rat-holing), the effective volume decreases, increasing refill frequency unexpectedly.
Minimum Weighment: The usable batch weight must be large enough for the load cells to resolve accurately. Very short cycles reduce accuracy.
Frequently Asked Questions (FAQ)
What is the ideal refill frequency for a loss-in-weight feeder?
Generally, a refill cycle every 10 to 60 minutes is acceptable. If refilling occurs every 2 minutes, the system spends too much time settling and refilling, reducing overall accuracy.
Why is the refill rate important?
During refill, the feeder locks the screw speed (volumetric mode). If the refill takes too long, the feeder cannot correct for density changes, leading to feed rate errors.
Can I use this calculator for liquid loss-in-weight?
Yes, the math is identical. Simply use the liquid's density and the tank volume. Ensure the units (kg/L) are consistent.
What happens if the refill rate is lower than the feed rate?
The hopper will never fill up. The system will eventually run empty and alarm. The refill source must always exceed the target feed rate.
How do I calculate bulk density?
Weigh a known volume of material. Formula: Density (kg/m³) = Weight (kg) / Volume (m³). Note that 1000 Liters = 1 m³.
What is "Turndown Ratio"?
It is the ratio between the maximum and minimum controllable feed rates. A high turndown ratio means the feeder is flexible for different products.
Does this calculator account for "heel" weight?
The "Low Level Setpoint" acts as the heel (material left in the bottom). We calculate usable weight based on the difference between High and Low setpoints.
Why does the weight reading fluctuate?
Vibration, air currents, or flexible connections that are too stiff can cause weight signal noise. Digital filtering in the controller usually handles this.